For processing workpieces by a machine for thermal welding or cutting, in particular a laser processing machine, splashes and smoke are produced in the processing region of the workpiece. The splashes and smoke can deposit on an optical surface, facing the workpiece, of an optical element of the processing head of the machine and soil the optical element. Splashes can burn into the optical surface. Smoke is deposited on the optical surface and can be removed from the optical surface to a certain degree. Deposits or damage to the optical surface produce increased absorption of laser radiation. Consequently, the thermal load of the optical element is increased which produces in the end a noticeable reduction of the laser performance available in the processing region. For heavy soiling, in particular through splashes, the increased absorption of laser radiation could destroy the optical element.
Consequently, early detection of splashes or smoke on the optical element is required to ensure constant thermal output in the working region of the workpiece and prevent destruction of the optical element by switching off the machine in due time for thermal processing, or when cleaning or replacement of the optical element is indicated.
Japanese patent number JP 01186296 A, discloses detection of a deflection or scattering of laser beams by means of detectors disposed on the side of the optical element.
German patent number DE 198 39 930 C1, describes the use of an additional light source with a light beam coupled on one side of the optical element and exiting on the other opposite side of the optical element. The intensity of a light beam of the additional light source is measured like a light barrier. A decrease in intensity means increasing thermal load of the optical element. This permits detection mainly of inner material defects such as cracks.
Further conventional measures include monitoring of the start of destruction of the optical element through an optical signal (light flash) according to European patent number EP 0 988 916 A1, or by an acoustic signal (cracking) according to German patent number DE 196 36 249 A1.
Conventionally, the thermal radiation emitted by the optical element may be detected (see, e.g., DE 3807873). However, the protecting glass may be destroyed before the temperature detector reacts due to the poor thermal conductivity of glass.